1. Field of the Invention
This invention relates to protective relays for electric power distribution systems, and in particular, to an electronic protective relay having an overcurrent trip characteristic which allows its operation to be coordinated with the trip characteristics of prior art electromechanical protective relays found in existing electric power distribution systems.
2. Background Information
Typically, the trip functions of the protective relays controlling the response of a hierarchy of circuit breakers in an electric power distribution system are coordinated so that the breaker closest to the cause of the overcurrent is tripped first to isolate the fault and limit disruption in the system. Coordination is implemented by incorporating a time delay in the trip functions of the upstream protective relays in order to provide an interval for the downstream relays to respond first. If the downstream relay does not respond or the fault is above the downstream protective relay, a trip of the upstream circuit breaker is initiated when the delay times out.
Traditionally, electromechanical relays have been used in such applications. The electromechanical relays provide a delayed trip which is an inverse function of time. A common inverse function is the I.sup.2 t function which is a measure of heat generated by an overcurrent condition, although other inverse functions of the general form I.sup.p where the power P can be less than, equal to, or more than 1, are used. In response to an overcurrent above a pickup value, the electromechanical protective relays integrate the selected inverse current function over time and generate a trip signal when the integrated value reaches a selected limit. The physical characteristics of the electromechanical protective relays have the effect of introducing an additional, fixed time delay into the response to an overcurrent. They also have a delay in resetting the integrated value when the current falls below the pickup value.
Electronic protective relays are replacing electromechanical relays for these applications. Typically, the electronic protective relays utilize a microprocessor to implement the selected, or selectable inverse current function. As many systems still incorporate electromechanical protective relays, it is required that the electronic protective relays emulate the electromagnetic relays to assure effective coordination between the two types of relays. Thus, the American National Standard Institute (ANSI) and the International Electrotechnical Commission (IEC) have both established the following response curve to be implemented by the electronic relays: ##EQU1##
Where: T=Trip Time in seconds; D=Time Multiplier Setting; I.sub.pu =Pickup Current Setting; A=Variable Time Delay Setting; B=Fixed Time Delay
As can be seen from Equation 1, precise application of this relationship requires a division operation which is computationally burdensome. As a microprocessor is typically performing additional functions such as metering, it is desirable to avoid the need to implement the division function. An approximation of the above function can be generated by just summing the values of I.sup.p for each digital sample properly scaled and adding in the fixed time B. This will work reasonably well were the overcurrent magnitude remains fairly constant which is reasonable in some industrial applications. However, in situations where the fault current changes appreciable in magnitude, significant error results from using this technique. Such changes in fault current can occur, for instance, where the impedance of a fault varies such as when a tree limb blows against a conductor, or where there is an inappropriate response by another protective relay in the system.
There is a need, therefore, for an improved protective relay for an electric power distribution system.
More particularly, there is a need for an improved electronic protective relay which can better emulate an electromechanical relay, but without an excessive computational burden or error.